Whale shark

The whale shark, Rhincodon typus, is a slow-moving filter feedingshark, the largest living fishspecies. The largest confirmed individual was 12.65 metres (41.50 ft) in length. The heaviest weighed more than 36 tonnes (79,000 lb),[2] but unconfirmed claims report considerably larger whale sharks. This distinctively-marked fish is the only member of its genusRhincodon and its family, Rhincodontidae (called Rhinodontes before 1984), which belongs to the subclass Elasmobranchii in the class Chondrichthyes. The shark is found in tropical and warm oceans, lives in the open sea with a lifespan of about 70 years.[3] The species originated about 60 million years ago. Although whale sharks have very large mouths, they feed mainly, though not exclusively, on plankton, microscopic plants and animals, although the BBC program Planet Earth filmed a whale shark feeding on a school of small fish.[4]

The species was distinguished in April 1828, following the harpooning of a 4.6-metre (15.1 ft) specimen in Table Bay, South Africa. Andrew Smith, a military doctor associated with British troops stationed in Cape Town described it the following year.[5] He published a more detailed description in 1849. The name "whale shark" comes from the fish's physiology; as large as a whale, it too is a filter feeder.

Anatomy and appearance

As a filter feeder it has a capacious mouth which can be up to 1.5 metres (4.9 ft) wide and can contain between 300 and 350 rows of tiny teeth.[6] It has five large pairs of gills. Two small eyes are located towards the front of the shark's wide, flat head. The body is mostly grey with a white belly; three prominent ridges run along each side of the animal and the skin is marked with a "checkerboard" of pale yellow spots and stripes. These spots are unique to each individual and are useful for counting populations. Its skin can be up to 10 centimetres (3.9 in) thick. The shark has a pair each of dorsal fins and pectoral fins. Juveniles' tails have a larger upper than lower fin while the adult tail becomes semi-lunate (crescent-shaped). Spiracles are just behind the eyes.

The whale shark is not an efficient swimmer since it uses its entire body, unusual for fish, to attain an average speed of around 5-kilometre-per-hour (3.1 mph). The largest specimen was caught on November 11, 1947, near Baba Island, in Karachi, Pakistan. It was 12.65 metres (41.50 ft) long, weighed more than 21.5 tonnes (47,000 lb), and had a girth of 7 metres (23.0 ft).[7] Stories exist of vastly larger specimens — quoted lengths of 18 metres (59 ft) are not uncommon in the popular shark literature — but no scientific records support their existence. In 1868 the Irish natural scientist Edward Perceval Wright obtained several small whale shark specimens in the Seychelles, but claimed to have observed specimens in excess of 15 metres (49.2 ft), and tells of reports of specimens surpassing 21 metres (68.9 ft).

In a 1925 publication, Hugh M. Smith described a huge animal caught in a bamboo fish trap in Thailand in 1919. The shark was too heavy to pull ashore, but Smith estimated that the shark was at least 17 metres (56 ft) long, and weighed approximately 37 tonnes (82,000 lb), which have been exaggerated to a more precise measurement of 17.98 metres (58.99 ft) and weight 43 tonnes (95,000 lb) in recent years. A shark caught in 1994 near Tainan in Southern Taiwan reportedly weighed 35.8 tonnes (79,000 lb).[8] There have even been claims of whale sharks of up to 23 metres (75 ft). In 1934 a ship named the Maurguani came across a whale shark in the Southern Pacific Ocean, rammed it, and the shark consequently became stuck on the prow of the ship, supposedly with 4.6 metres (15.1 ft) on one side and 12.2 metres (40.0 ft) on the other.[9] No reliable documentation exists for these claims and they remain "fish-stories".

Diet

The whale shark is a filter feeder — one of only three known filter feeding shark species (along with the basking shark and the megamouth shark). It feeds on macro-algae, plankton, krill, Christmas Island red crab larvae,[10] and small nektonic life such as small squid or vertebrates. The many rows of teeth play no role in feeding; in fact, they are reduced in size in the whale shark. Instead, the shark sucks in a mouthful of water, closes its mouth and expels the water through its gills. During the slight delay between closing the mouth and opening the gill flaps, plankton is trapped against the dermal denticles which line its gill plates and pharynx. This fine sieve-like apparatus, which is a unique modification of the gill rakers, prevents the passage of anything but fluid out through the gills, trapping anything above 2 to 3 millimetres (0.079 to 0.12 in) in diameter. Material caught in the filter between the gill bars is swallowed. Whale sharks have been observed "coughing" and it is presumed that this is a method of clearing a build up of food particles in the gill rakers. Whale sharks migrate to feed and possibly to breed.[3][11][12]

The whale shark is an active feeder, targeting concentrations of plankton or fish. It is able to ram filter feed or can gulp in a stationary position. This is in contrast to the passive feeding basking shark, which does not pump water. Instead, it swims to force water across its gills.[3][11]

Behaviour toward divers

Despite its size, the whale shark does not pose significant danger to humans. Although massive, whale sharks are docile fish and sometimes allow swimmers to hitch a ride.[13] Whale sharks are actually quite gentle and can play with divers. Divers and snorkelers can swim with this giant fish without risk, apart from unintentional blows from the shark’s large tail fin.[14]

Whale sharks in captivity

Two whale sharks are featured as the main attraction of Osaka Aquarium Kaiyukan and as of 2005, three whale sharks are in captivity at the Okinawa Churaumi Aquarium in Japan. The Ioworld Aquarium in Kagoshima, Japan also features a single adult whale shark as a major attraction. One is also on display in the Taiwan, KentingNational Museum of Biology and Aquarium. Four whale sharks, two males, Taroko, and Yushan,[15] and two females, Alice and Trixie, live in the Georgia Aquarium, in Atlanta, USA. Two male whale sharks, Ralph and Norton, died in captivity at the Georgia Aquarium on January 11, 2007 and June 13, 2007 respectively. The two females were added on June 3, 2006 in hopes that reproduction in whale sharks could be studied in captivity. All six whale sharks were imported from Taiwan, where whale sharks are dubbed tofu sharks because of the taste and texture of the flesh. Two whale sharks live at Polar Ocean World in Qingdao, China. One whale shark was at the Atlantis Hotel in Dubai, but was released in March, 2010.[16] One whale shark lives in Dalian, China.

Reproduction

The capture of a female in July 1996 which was pregnant with 300 pups indicates that whale sharks are ovoviviparous.[3][17][18] The eggs remain in the body and the females give birth to live young which are 40 to 60 centimetres (16 to 24 in) long. It is believed that they reach sexual maturity at around 30 years and the life span is an estimated 70[3] to 100 years.[19]

On March 7, 2009, marine scientists in the Philippines discovered what is believed to be the smallest living specimen of the whale shark. The young shark, measuring only 38 centimetres (15 in), was found with its tail tied to a stake at a beach in Pilar, Philippines, and was released into the wild. Based on this discovery some scientists no longer believe that this area is just a feeding ground; it may also be that this site is a birthing ground as well.[20]

Conservation status

The whale shark is targeted by commercial fisheries in several areas where they seasonally aggregate. The population is unknown and the species is considered vulnerable by the IUCN.[1] In 1998, the Philippines banned all fishing, selling, importing and exporting of whale sharks for commercial purposes,[21] followed by India in May 2001,[22] and Taiwan in May 2007.[23]They are currently listed as a vulnerable species; however, they continue to be hunted in parts of Asia, such as Taiwan and the Philippines.

In 2006, Resorts World Sentosa announced its plans to bring in whale sharks for their marine life park. This was met with opposition from seven notable conservation societies. In 2009, the plan was shelved in favour of a search for other alternatives.[24][25]

In 2010, the Gulf of Mexico oil spill resulted in 4,900,000 barrels (779,000 m3) of oil flowing into an area south of the Mississippi River Delta, where one-third of all whale shark sightings in the northern part of the gulf have occurred in recent years. Sightings confirmed that the whale sharks were unable to avoid the oil slick which was situated on the surface of the sea where the whale sharks feed for several hours at a time. However, no dead whale sharks have been found.[26] .

In human culture

Snorkeling with whale shark.

Known as a deity in a Vietnamese culture, the whale shark is called "Ca Ong", which literally translates as "Sir Fish".

In Mexico, and throughout much of Latin America, the whale shark is known as "pez dama" or "domino" for its distinctive patterns of spots. However, they go by "Sapodilla Tom" in Belize due to the regularity of sightings near the Sapodilla Cayes on the Belize Barrier Reef.

In Africa, the names for the whale shark are very evocative: "papa shillingi" in Kenya came from the myth that God threw shillings upon the shark which are now its spots. In Madagascar the name is "marokintana" meaning "many stars".

Javanese also reference the stars by calling it "geger lintang," meaning "stars in the back". In the Philippines, it is called "butanding", while in Madurese, it is known as "kikaki".